Influenza viruses possess a number of mechanisms to elude host humoral immunity against hemagglutinin (HA) that mediates viral entry. Reassortment of genome segments can lead to antigenic shift while gradual accumulation of mutations leads to genetic drift (Desselberger U, et al. 1978, Schild G, et al. 1974). In addition to mutations in the antigenic epitopes of the HA that can impact the ability of pre-existing antibodies to recognize mutant HA, mutations in N-linked glycosylation of HA can promote viral evasion of antibody recognition by altering the oligosaccharide layer surrounding the HA (Schulze I T, 1997). Glycan residues can restrict the binding of some antibodies to their epitopes, leading to loss of antibody recognition and immunogenicity in a phenomenon known as glycan shielding (Wanzeck K, et al. 2011, Wei C-J, et al. 2010). Additionally, mutation and genetic drift preferentially occur at positions in the HA globular head that are not protected by glycans (Das S R, et al. 2010). Glycan residues also influence receptor binding and consequently can affect viral replication kinetics.
The evolution of influenza viruses frequently includes modification of the number and position of glycosylation sites (Long J, et al. 2011). For H3N2 viruses, the number of glycosylation sites has increased from two to ten over the last 40 years (4). However, increasingly glycosylated HA has been correlated with decreased virulence in mice and reduced viral fitness (Das S R, et al. 2011, Vigerust D J, et al. 2007). While increasing glycosylation can shield HA from neutralizing antibodies, the viral affinity for cellular receptors is obligatorily decreased (Abe Y, et al. 2004, Das S R, et al. 2011). Also, compensatory mutations in either the HA or neuraminidase (NA) are required to balance receptor binding and release activities (Wagner R, et al. 2000). These compensatory mutations in NA have been associated with the acquisition of natural resistance to NA inhibitors (Hensley S E, et al. 2011).
Numerous genetic and antigenically distinct lineages of swine influenza virus circulate concurrently in pigs, including β, γ and δ-cluster H1N1 and H1N2 viruses, as well as the H3N2 subtype (Lorusso A, et al. 2012). The δ-cluster was originally subdivided into two subclusters (δ-I and δ-II), however, more recent work demonstrated five distinct genetic subclusters (δ-A, δ-B, δ-C, δ-D, δ-E) representing at least three antigenically distinct groups (Hause B M, et al. 2011, Vincent A L, et al. 2009).
While killed viral vaccines are efficacious when the vaccine strains match the field challenge virus, the rapid mutation rate of influenza requires frequent strain changes to ensure genetic and antigenic match between the vaccine strains and circulating viruses. Additionally, the multiple subtypes and lineages co-circulating in swine frequently require five or more vaccine strains in order to include representatives of each type. A single live vaccine that elicits broad spectrum immunity offer a significant advance in swine health programs.